873 research outputs found
Dimensionality effects in dipolar fluids
Using classical density functional theory (DFT) in a modified mean-field
approximation we investigate the fluid phase behavior of quasi-two dimensional
dipolar fluids confined to a plane. The particles carry three-dimensional
dipole moments and interact via a combination of hard-sphere, van-der-Waals,
and dipolar interactions. The DFT predicts complex phase behavior involving
first- and second-order isotropic-to-ferroelectric transitions, where the
ferroelectric ordering is characterized by global polarization within the
plane. We compare this phase behavior, particularly the onset of ferroelectric
ordering and the related tricritical points, with corresponding
three-dimensional systems, slab-like systems (with finite extension into the
third direction), and true two-dimensional systems with two-dimensional dipole
moments.Comment: 7 pages, 2 figure
A minimal model for short-time diffusion in periodic potentials
We investigate the dynamics of a single, overdamped colloidal particle, which
is driven by a constant force through a one-dimensional periodic potential. We
focus on systems with large barrier heights where the lowest-order cumulants of
the density field, that is, average position and the mean-squared displacement,
show nontrivial (non-diffusive) short-time behavior characterized by the
appearance of plateaus. We demonstrate that this "cage-like" dynamics can be
well described by a discretized master equation model involving two states
(related to two positions) within each potential valley. Non-trivial
predictions of our approach include analytic expressions for the plateau
heights and an estimate of the "de-caging time" obtained from the study of
deviations from Gaussian behaviour. The simplicity of our approach means that
it offers a minimal model to describe the short-time behavior of systems with
hindered dynamics.Comment: 8 pages, 6 figure
Non-equilibrium condensation and coarsening of field-driven dipolar colloids
In colloidal suspensions, self-organization processes can be easily fueled by
external fields. One particularly interesting class of phenomena occurs in
monolayers of dipolar particles that are driven by rotating external fields.
Here we report results from a computer simulation study of such systems
focusing on the clustering behavior also observed in recent experiments. The
key result of this paper is a novel interpretation of this pattern formation
phenomenon: We show the clustering to be a by-product of a vapor-liquid first
order phase transition. In fact, the observed dynamic coarsening process
corresponds to the spindodal demixing that occurs during such a transitionComment: 6 pages, 5 figure
The effect of distance on reaction time in aiming movements
Target distance affects movement duration in aiming tasks but its effect on reaction time (RT) is poorly documented. RT is a function of both preparation and initiation. Experiment 1 pre-cued movement (allowing advanced preparation) and found no influence of distance on RT. Thus, target distance does not affect initiation time. Experiment 2 removed pre-cue information and found that preparing a movement of increased distance lengthens RT. Experiment 3 explored movements to targets of cued size at non-cued distances and found size altered peak speed and movement duration but RT was influenced by distance alone. Thus, amplitude influences preparation time (for reasons other than altered duration) but not initiation time. We hypothesise that the RT distance effect might be due to the increased number of possible trajectories associated with further targets: a hypothesis that can be tested in future experiments
Non-linear rheology of active particle suspensions: Insights from an analytical approach
We consider active suspensions in the isotropic phase subjected to a shear
flow. Using a set of extended hydrodynamic equations we derive a variety of
{\em analytical} expressions for rheological quantities such as shear viscosity
and normal stress differences. In agreement to full-blown numerical
calculations and experiments we find a shear thickening or -thinning behaviour
depending on whether the particles are contractile or extensile. Moreover, our
analytical approach predicts that the normal stress differences can change
their sign in contrast to passive suspensions.Comment: 11 pages, 10 figures, appear in PR
- …